21

Physics, Astronomy & Cosmology / Re: What is the cosmic microwave background?

« on: 08/10/2019 17:28:34 »

And just to add to What Kryptid already said. While the photons we detect today are different ones than the ones we detected yesterday or will detect tomorrow. The one we detected yesterday where a bit "younger" and and the one we will detect tomorrow will be a bit "older".   In other words, the CMB is still decreasing in temp as the universe continues to expand.  The decrease is very small, so it would take a long time before it became enough to be discernible by our measurement standards, but it exists.


 

So the radiation we saw last week must now be gone forever. It sounds like this radiation must have been tavelling past us and 'disappearing forever' for the last umpteen billion years. why is it STIIL travelling past us in a way that we can register with instruments?

This is like saying that because the average water molecule speed is something is in the 100's of meters per second, that a fish in a lake should not detect any water around him because the water molecules that he felt yesterday has traveled off to some other part of the Lake. New water molecules have replaced them.*

*Now it is true that with the lake being a confined body of fixed size, he can encounter the same water molecule more than once.  This is note completely inconceivable for the universe if it were of a finite fixed size.  a Photon could pass the Earth, "circumnavigate" the universe and pass it again. How long it takes between passes would depend on the size of the universe. ( in a really small "closed" universe you could conceivably look off into the distance with a telescope and see the back of your own head)
 But with an expanding universe, even if it is finite, the size and rate of expansion could be such that the "circumference" grows faster than light can traverse it, and light passing you at any given moment can never return. ( but this still doesn't mean that it won't be replaced by a different photon.)

22

Physics, Astronomy & Cosmology / Re: What is the cosmic microwave background?

« on: 08/10/2019 16:02:23 »

The problem is that you are thinking of the Big Bang as an explosion in space, rather than the expansion of space.
With an explosion in space, the Photons could have raced past the matter into the space beyond. But with the Big Bang, there is no "space beyond" for the photon to expand into. We are at the "forefront" of this expansion.
An analogy would be that the universe is like the skin of an expanding soap bubble. Everything in it, photons, matter, etc. is confined to the skin. When the bubble is small everything is much more crowded together and as it expands, Things on a whole move apart and it is less crowded.  With photons, when it is small their frequencies are high ( the universe is "hot"), but as the bubble expands, they are stretched out to longer wavelengths and lower frequencies ( the universe "cools"). The CMB are those early photons that have been stretched out, but still confined to the same "Bubble skin" as everything else.

23

Physics, Astronomy & Cosmology / Re: Why are my atoms not flying apart? Please Hurry - May Have Limited Time Left

« on: 21/09/2019 18:14:23 »

Okay, I'm not really worried about disintegrating.  Just wanted to get your attention.  I asked the following question on a more general forum and got a single, not-so-satisfactory answer.  So here goes:

If the very fabric of space is expanding, then I would assume that such expansion is resulting in increasing distances between all physical objects -- not only macroscopic objects like galaxies, but also between the very particles that make up atoms.  I get that at that scale, such increases in distance would be infinitesimally minuscule -- but I also assume not zero.

My layperson understanding of particle physics tells me that the strength of the forces governing subatomic interactions are very critical -- and measured to many, many decimals of precision.  Furthermore, if the constants were reduced by the even the slightest amount, one result would be that nucleonic particles would not be able to adhere (or whatever the term of art is) to form stable nuclei.

So (I'm sure you see where this is going), if the forces have remained constant, and the distances, however infinitesimally, are increasing, then at some point in the expansion of the universe, won't atoms cease to exist?

As of this time, Atomic radii are not increasing due to the expansion of the universe.  The bonds holding them togehter are stronger than the effect caused by the expansion. 
To explain, I'll use a somewhat loose analogy.   Imagine you and someone else are standing on a waxed tile floor in your stocking feet,  each standing in the center of am adjacent tile.  The tiles all begin to expand.  You and your friend will move apart. But if you grip hands, the floor will still expand but you won't move apart.  The strength of your grip is stronger than the friction between your socks and floor.  This is somewhat similar to what is happening with the universe expansion. The expansion, if it could, would separate adjoining atoms, but the force holding the atom together holds them in place.
This is true not only for atoms, but galaxies and groups of galaxies.  The gravitational bound is stronger than the effect of the expansion and holds them together against it and keeps them from expanding along with the universe.  Galaxies and groups of galaxies retain their size, while moving apart from other galaxy groups.

Now, having said that, this may not always be the case.  A while back we learned that the expansion of the universe is accelerating and the that the rate at which it expands is increasing.
So let's go back to the tiled floor.  If we keep increasing the rate at which the floor expands, the tendency for the soles of your feet being dragged along increases.  Eventually, it will exceed the strength of your grip and you will be pulled apart.
In cosmology, this eventual result due to an ever increasing rate of expansion is call the "Big Rip".
As the rate continues to increase, the ability for structures to hold together against it begins to fail.  First groups of galaxies will begin to pull apart,  then later galaxies themselves will pull apart into independent star systems. Down the road, the planets of those star systems will be ripped away.   Going further forward more, the planets and stars will be ripped apart as their gravity will no longer be enough to hold them together.  This will continue as smaller and smaller structures succumb.

If this turns out to be the eventual fate of the universe, it turns out that the end comes fairly quickly. For one model, which has the end coming in 22 billion years, galaxies would be pulled apart ~60 million years before the end, and our solar system would hold together until just three months until the end, planets and stars wouldn't go until minutes before the end, and atoms just moments prior to the end.

And we really don't know if the the Big Rip scenario is the actual fate of the universe.

24

Chemistry / Re: Where does the spent car tire rubber go?

« on: 06/09/2019 17:47:46 »

One thing to keep in mind is that the rubber will decompose over time.   A car tire will completely decompose in 50-80 yrs.
But this doesn't mean that there should still be 50-80 yrs worth of accumulation of worn tire rubber left around.  That 50-80 yr figure is for a tire that is still one large single lump of rubber not already broken up into innumerable small particles.
An analogy is like dropping a salt lick block whole into a lake and grinding it to a powder first and scattering it over the whole lake. The block will take a lot longer to dissolve than the powder will.

Stupid question but micro plastics hang around and never completely disappear. They apparently wind up back in the food chain. Does rubber and plastic not decompose in a similar way, do we have micro rubbers:) in the food chain as well as plastics.

No, rubber and plastic do not decompose the same way, not even all plastics decompose the same.  As stated, an automobile tire will decompose entirely in 50-80 years. A plastic bottle will take ~450 yrs to decompose. Styrofoam doesn't decompose at all.  So it is Styrofoam and like materials that produce the "everlasting" micro-plastics, not all plastics.  That is not to say you can't have micro-particles of rubber or plastics in the food chain, especially if we are producing the materials at a faster rate than they decompose. 

25

Physics, Astronomy & Cosmology / Re: How Can This Black Hole Be So Big Ton 618 ?

« on: 21/08/2019 16:33:12 »

See: https://en.wikipedia.org/wiki/Supermassive_black_hole#Formation

If black holes evaporate via Hawking radiation, a supermassive black hole with a mass of 1011 (100 billion) M☉ will evaporate in around 2×10^100 years . Would galaxies drift apart without their blackholes??

Estimated mass of SMBH at center of our galaxy: 4 million solar masses.
Total estimated mass of our galaxy 700 billion solar masses.
So the SMBH only makes up 0.0006% of the mass of the galaxy.  It's loss would have an insignificant effect on the dynamics of the galaxy.
For comparison the Sun converts about the same percentage of its mass to light in just under 1,000,000 years.  Ergo, A galaxy would loose mass via radiation loss at a faster rate than it would by SMBH evaporation, even if the SMBH were free to evaporate. 
Secondly, For a BH to even lose net mass via evaporation, It would need to lose energy/mass at a faster rate than it takes in.  Matter, starlight, and even the Cosmic background radiation all all sources which will feed the BH. 
The basic rule is that the Hawking temp of the BH has to higher than the average ambient temp of its surroundings.  Even if we remove all other sources, the CMBR still remains.  Hawking Temp goes down with size, and beyond a certain size, a BH will be below the CMBR temp.    This mass is approximately equal to that of our moon.

Ergo, before any SMBH at the center of any galaxy can even start to experience a net loss of mass via Hawking radiation, the universe will have to have expanded and cooled by a great deal, the galaxies would have had to used up their source of hydrogen for forming new stars, and remaining husks of old stars will have to have cooled significantly.( considering the fact that small red dwarfs can burn for trillions of years, and It can take a quadrillion years (or longer) for the white dwarf formed at the end of solar mass range stars to cool to black dwarf stage, you are going to be for a long wait.   If the "Big Rip" hypothesis is correct, run away expansion of the universe would have torn galaxies, star systems and even stars themselves apart before then, so it would be a bit of a moot point.

26

Physics, Astronomy & Cosmology / Re: Where were the electrons before the big bang?

« on: 22/06/2019 15:54:14 »

During the earliest stages of the universe, the fundamental forces were unified into a single force( or at least were all of the same strength).  This means that they did not even exist in forms that they have today.  As the universe cooled, different forces separated out from that mix, one by one. First gravity, then the strong force...   Thus when the "quark soup" was in sway, the electromagnetic force had yet to come into existence as a singular force, so the entity we now call the electron wasn't possible.
To use an analogy, its like when you freeze water.   It doesn't really make sense to ask what "form" or shape the crystal structure of ice took before the water started to freeze, as the crystals depend on the water having the properties to allow them to form.

27

Physics, Astronomy & Cosmology / Re: Do Peking and Helsinki lie on a straight line through Earth's core?

« on: 22/06/2019 01:16:02 »

The red line represents the straight-line path.  The yellow sphere represents the extent of the outer core.


core.png (122.53 kB . 960x540 - viewed 1918 times)

So the question is, What do you mean by "pass the core"?   Do you mean "pass through" the core, or do you mean "bypass" the core without entering it?  An if you mean bypass, do you mean it has to be close miss or does any miss count?  It is obvious that the line never enters the outer core, nor does it come very close.  so the answer to your question depends on exactly what the question is. 

28

Physics, Astronomy & Cosmology / Re: How much does the moon weigh?

« on: 08/06/2019 22:33:01 »

I was asked on the radio this week "how much does the Earth weigh?"

More accurately, we were of course discussing the mass of the Earth, and the answer, which I happened to know, is about 6 x 10²⁴kg.

Later, driving to work, I began to think - as you do - what I would have said if the questioner had enquired instead about the mass of the Moon.

Naively, I reasoned that gravity is about 20% of that felt on the Earth's surface, so the Moon must have a mass 20% that of Earth.

Intrigued, I later looked it up. The stated mass of the Moon is about 7 x 10²²kg. That's only about 1% of the mass of the Earth.

Is the acceleration due to gravity on the lunar surface as large as it is (almost 20% of the Earth's despite the lunar mass being only 1% of Earth mass) because the Moon is correspondingly smaller, so the inverse square law being what it is (GmM/r²) it works out that way?

Lazily, I've not done the maths to check...

Assuming a constant density, Mass goes up by the cube of the radius, and since acceleration due to gravity decreases by the square the distance, surface gravity would increase at the same rate as the radius.
The Moon's radius is 1738 km, ad the Earth's is 6378.   6378/1738 = ~3.7,  so you might expect surface gravity on the Earth to be 3.7 times that of the Moon. However, the Moon is not as dense as the Earth, ( the Earth is roughly 1 2/3 times more dense.   3.7 * 5/3 = 6    The Moons surface gravity is roughly 1/6 that of the Earth's.

29

Physics, Astronomy & Cosmology / Re: Proxima Centauri b and the red dwarf?

« on: 01/06/2019 16:25:50 »

If Proxima b is bigger than the earth, it may have a bigger magnetosphere, could this trap water?

We can compare Mercury and Venus, the latter is close to the Sun and has a very thick atmosphere. Smaller than the Earth, yet inside maybe more Iron?

The size of a planet has nothing to do with its magnetic field, which is generated by its spinning core.  One of the issues with orbiting so close to the star is that the planet is going to be tidally locked so that it rotation and orbit have the same period. Since Proxima b takes about 11 days to orbit, it would also take 11 days to rotate. The slower rotation doesn't bode well for generating a strong field. 
It is estimated that  Proxima loses mass to solar wind at a rate about 1/5 of that of our Sun, but Proxima b is only about 1/20 of the distance from its star than the Earth is from the Sun. This ends up with the Solar wind being more than 80 times stronger for Proxima b than it is for the Earth. 

While this does not completely rule out Proxima b from having a viable biosphere, it does but it at very long odds.

30

Physics, Astronomy & Cosmology / Re: The Northern Stars and Exoplanets

« on: 31/05/2019 18:02:37 »

The present Northern star Polaris is actually a triple star system. the Primary star is a yellow giant some 2000 times brighter than our Sun, which also is a Cepheid variable, meaning its output varies over a set period, varying by about 8% over 4 days.  Its overall luminosity is also slowly increasing, having increased by ~2.5 times in the last 2000 yrs or so. 
The other two stars are also brighter than our sun, but only by about 3-4 times.
The main star is not a good candidate for planets due to its unstable nature, and the smaller of the other two is too close to the main star.   This leaves one star that could even possibly have some type of planet system.   However,  larger more massive stars spent less time in the main sequence. This star would only be expected to do so for about 1.5% as long as our sun.  Likely too short a time for any complex lifeforms to form.
Kochab, the "old" northern star is also more massive and brighter than our Sun.  It has also reached that part of its life cycle where it is beginning to expand into a giant.  Any planets it may have had are cooked by now.

31

Physics, Astronomy & Cosmology / Re: How do galaxies keep a stable size?

« on: 23/05/2019 16:40:07 »

Individual galaxies do not expand due to the expansion of space because they are compact enough that the mutual gravitational attraction holds them together. This effect actually extends all the way out to include groups of galaxies.  The individual galaxies of our local group are not receding from each other but orbit each other as a gravitationally bound system.  It the larger groups of galaxies that are moving apart from each other.

The individual stars in a galaxy are in orbit around the their mutual center of gravity.  Orbits are not as fragile as some people tend to think they are.  If you slow down or speed up an object in orbit, it will not necessarily "fall out" or "fly off" of orbit.  Unless you make a large change in its speed, it just settles into a new orbit.   So for example,  if you wanted to make the Moon "fall" into the Earth,  you would have to reduce its orbital speed to less than 1/5 of its present orbital speed, and to make it fly away into space, you would have to increase it by almost 41%.   
Galaxies are made of stars that fall somewhere between.  It's a bit more complicated with galaxies, but the general idea is the same. Stars in the galaxies don't have to maintain a delicate balance to remain in orbit and have a fair amount of "wriggle room". *
This is not to say that galaxies don't evolve over time.  Early in their lifetimes, things are a lot less stable.   You still had material falling in towards the center feeding the black hole their, etc.   And even later, and occasional close encounter will fling a star clear of its parent galaxy.  But the vast majority of stars in a mature galaxy are in that "comfort zone" as far as their orbit goes, and would take quite a bit to knock them completely out of it.

* As an analogy, think of a rocking chair.  If you give it a push, it will rock back and forth, but won't fall over. It takes quite a large shove to make it completely topple over.

32

Physics, Astronomy & Cosmology / Re: Is spacetime always understood in relation to the speed of light (em radiation)?

« on: 19/05/2019 16:26:18 »

Einstein argued that if you were inside a box you would not be able to tell if you were in a freely falling frame in a gravitational field or in an inertial frame. In an accelerating frame an object would be emitting electromagnetic radiation. So does the free falling object in the box emit electromagnetic waves? Does the radiation increase with increasing speed? If it emits no radiation due to acceleration then it can be considered truly inertial. In which case Einstein is correct. Which is it?

https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charged_particles_in_a_gravitational_field

33

Physics, Astronomy & Cosmology / Re: Does a photon self propagate due to the interactions of the electric and magnetic fields?

« on: 15/05/2019 16:26:35 »

To add to what Colin2B has already said.  A photon, being a quantum of electromagnetic radiation, is the smallest divisible unit of radiation energy you can have for EMR of a given wavelength. 

And, as pointed out, if a single photon strikes photographic paper, it produces a single localized spot. ( like a particle would).
However, if you send a series of single photons, one at a time, through a double slit interferometer, While each individual photon will produce a localized dot on the paper,   The pattern formed after many photons have passed through will be an interference pattern like you would expect from a wave.
A classical particle would not create the interference pattern,  while with a classical wave, you'd get the interference pattern, but it wouldn't be formed from individual dots.( the whole pattern would form at once and just get more distinct as more light passed through the slits. 
While you will hear it said that exhibits both the properties of particles and waves, it is probably more accurate to say that the classical distinction between wave and particle just doesn't hold, and that this becomes more apparent at the quantum level.
This characteristic is not limited to light alone.  Electrons( original considered classical particles) show the same behavior.  You can pass them through a diffraction grating and produce an interference pattern composed of individual dots also.
It is something built into the universe at that scale and not something exclusive to this or that "particle". 

34

Physics, Astronomy & Cosmology / Re: How do black holes evaporate?

« on: 17/04/2019 16:36:06 »

As far as I understand the popular science explanation of black hole evaporation, quantum fluctuations of vacuum at the event horizon create a pair of matter-antimatter particles. One escapes thus reducing the mass of the black hole. So far so good. However, just outside the horizon, the same effect must be continually feeding new particles into the black hole. The outside sphere is somewhat bigger, there must be more mass entering than leaving.

I suspect he popular explanation must be leaving out some important details?

One thing to keep in mind is that the formation of virtual particles just outside the event horizon comes from "borrowed" energy. Under normal circumstances, these particles only exist briefly, before rejoining and returning this borrowed energy. It is a quirk of QM that allow this to happen without violating energy conservation. 
If, however, one of the particles crosses inside the event horizon before the rejoining can occur ( it does not matter whether it is the particle or anti-particle),  You are left with a lone particle.  The energy books must be balanced in order for this particle to continue to exist, and this come from the mass of the BH.  One of the virtual particle pair has become a real particle at the expense of the BH.  This particle may escape, or more likely interact with another particle which produces light radiation that does escape the region near the event horizon. You end up with some "leakage" of radiation.
That being said, only black holes below a certain size would end up experiencing net shrinkage through Hawking radiation.  One of the features of Hawking radiation is that it increases as the mass of the Black hole decreases. Large black holes lose mass through Hawking radiation at a much lesser rate than they gain it from other sources.  Any stellar sized black hole, even if far away from any matter that it could absorb would still be gaining mass from just the cosmic background radiation faster than it lost it through Hawking radiation.   
For a black hole to lose mass faster than it gains it from the CMBR, it could not be any more massive than our Moon.
This is much smaller than any black holes that form today, but there is some speculation that small or "quantum" black holes could have formed during the early universe.  It is possible that some of them survived to our time.

Even present day large black holes are subject to eventual evaporation.  As the universe continues to expand, it cools which decreases the intensity of the background radiation.  At some point it will cool enough for stellar mass black holes to begin to evaporate as they lose mass faster than they gain it even through the CMBR.   But we are talking about really, really, really long time scales.

35

Physiology & Medicine / Re: Why do optometrists dilate our eyes then do the exam?

« on: 05/04/2019 16:47:40 »

The wider your pupils are, the shallower your depth of field.  Depth of field is the distance range over which objects in your vision will appear relatively in focus. With your pupils contracted and focused on an object 10 ft away, objects 7 to 13 ft away might look in focus, but with them dilated, it might drop to objects between 9.5 and 10.5 ft 
 By dilating your eyes and reducing your depth of field, they are making your eyes more sensitive in small changes in the prescription.  In other words, when they ask you if  "1" or "2" looks better, You'll see a larger difference with your eyes dilated than with them not, which allows them to fine tune the prescription more accurately.

36

Physics, Astronomy & Cosmology / Re: What would air and water do on a rotating space craft?

« on: 03/04/2019 20:43:49 »

I think your example is missing a step, overcoming inertia. A spinning space craft must constantly overcome inertia, which is especialy important with liquid.

When a drop of water hits a water surface another drop of water is released from the spash. On Earth gravity will bring the drop down, but in the space craft lake the drop will continue upward for several minutes. The wayword drop will continue until it hits something or dissipates.


www.maxpixel.net-Nature-Water-Drops-Of-Water-Liquid-578897.jpg (49.9 kB . 640x426 - viewed 1796 times)

But that drop of water will have the same tangential momentum it had when it it was part of the body of water.  Thus it will travel in a straight line along a vector determined by its initial inwards velocity and its tangential velocity.  Since the surface of the space ship is curved inwards as is the surface of the water, this path will intersect the water again.   For anyone moving along with the rotating craft, will also have a tangential speed that stays pretty much equal to that of the drop of water. So from their perspective, the water rises from the water and then falls back down.  There will be a small apparent drift due to Coriolis effect, the larger the "inward" velocity of the drop, the more apparent this will be.   But unless you completely remove all of the tangential motion of the drop ( effective throwing "backward" against the rotation at exactly the rotation speed), it will not fail to return to the water surface. 

37

Physics, Astronomy & Cosmology / Re: What would air and water do on a rotating space craft?

« on: 30/03/2019 22:44:12 »

Water would be 'down' just like water on Earth which is attracted to Earth by gravity.  So a spinning object would have 'down' feel like the outside of the craft and 'up' be towards the center of rotation.  The closer to the edge you get, the stronger the artificial gravity.  Thus there would be no 'gravity' at the center.

Thank you for answering. This is a good description of it works in movies, but I don't believe this is correct. While gravity has an area of effect, a spinning space craft can only effect what it touches. Without gravity a thrown baseball will continue it's trajectory until it is acted on by an outside force, even if that force is air. Throw that ball in the exact opposite of the rotation and it might stay in one spot. I don't know what the answer to this question is, which is why I'm asking.

If you threw the ball at exactly the right speed, then yes, you could make it standstill relative to the station axis and not "fall to the ground.  However the station would be still turning, Thus for you the person throwing the ball, it would appear that the ball was traveling in a circle at a constant distance from the floor.( ignoring any air resistance).
If you could ignore air resistance and any obstacles in the way, if you could throw a ball at ~7.9 km/sec in the horizontal while standing on the Earth, it would travel in circle around the Earth maintaining a constant distance from the ground. It would be in orbit around the Earth.   Throwing the ball at just the right velocity on the station is the equivalent putting it in orbit from the rotating station's frame of reference.*

But you can't ignore air resistance.  If add air to the station, that air will expand to fill the station, when it does so, it comes in contact with the rotating walls air molecules striking the wall will be be given some of the wall's these molecules will go on to transfer it to other molecules, etc.  Eventually, you will end up with the air in the station having the same angular velocity as the walls, and rotate with it.   If you throw your ball now, it will have to deal with the drag caused by this air which worls to drag the  ball around it with the station.  The ball will not be able to maintain is fixed position and will drift, until it hits the "floor" picking up more motion from it.   

The same happens for water, it would be eventually dragged to have the same velocity as the floor.  So while there might be a bit of "settling down" to do at first, eventually you'll end up with water hugging the floor and air pressing against it.

*This only works if you throw the ball at precisely the right speed. Any slower or faster and it will drift in a straight line until is hits the curved floor of the station.

38

Physics, Astronomy & Cosmology / Re: Do you think Tau Ceti e is inhabited?

« on: 10/02/2019 18:58:47 »

Call it 99%. 99% of 150° is  148.5°.

Sorry to be the pedant, but are you scaling a temperature? In ° F?!?

150° F

At the very least, please use an absolute temperature scale (Kelvin or Rankine) before multiplying temperatures by any factor, otherwise we end up with such silly results, like: "50% hotter than 32 °F (0 °C) is 48 °F (8.9 °C), but 50% hotter than 0 °C is, still 0 °C?" or even worse: "50 % hotter than 20 °F (–6 °C) is 30 °F (–1.1 °C)..."

Even with absolute temperatures, scaling is somewhat nonsensical, but at least the maths work out.

Yeah your right.  Silly mistake on my part.


  Using the same numbers gives ~ 144° F.  For difference of 6°F.  But then again, I was being generous with the 99% estimate.   At the highest resolution of the graph in the paper, you might get 99% of circular orbit equilibrium temp at an eccentricity of 0.4, better than twice the eccentricity of Tau Ceti e, and its on a curve that flattens out as you approach 0 eccentricity. 

So despite my embarrassing lapse, I still don't think that this is enough to significantly increase the chances of Tau Ceti e being habitable

39

Physics, Astronomy & Cosmology / Re: What is the origin of Earth's magnetic field?

« on: 09/02/2019 23:06:05 »

It is a mystery why Earth has a magnetic field, and Mars doesn't (any more).
Mars is slightly smaller than Earth, so it would have cooled slightly quicker - but not much quicker.

Perhaps the collision that produced the Moon might have injected a pulse of energy into the Earth's interior, keeping it hotter for longer? The iron core of the impactor would have sunk towards the center of the earth, merging with Earth's core.
See: https://en.wikipedia.org/wiki/Giant-impact_hypothesis

Earth's radius is ~1.87 times that of Mars,  This gives it ~3.5 times the surface area, but  ~6.5 times the volume of Mars.  Earth also has a density of 5.52 g/cc to Mars 3.93g/cc,  which makes the Earth over 9 times more massive.   All in all, Earth's mass to surface area ratio is ~2.66 times that of Mars.   Surface area determines how fast the heat can radiate away and mass determines the total heat contained for objects of the same temperature and like make up.  Thus Mars should be expected to lose its heat fairly quickly compared to the Earth

40

Physics, Astronomy & Cosmology / Re: How long could a 400kg space probe last in an antimatter plasma?

« on: 08/12/2018 22:09:15 »

Thanks Evan, Is there any sign of a signal yet as I think 4 December was mentioned as date of expected reconnection??

Perihelion took place on on Nov 5, and the probe passed it first post perihelion health test on Nov 16.
It is presently heading away from the Sun before looping back for the next perihelion pass in April

Here is a link showing where the probe is at this time:

http://parkersolarprobe.jhuapl.edu/The-Mission/index.php#Where-Is-PSP